Beautiful Patterns In Debris Disks Can Be Forged By Starlight

An artist's impression of a protoplanetary disk, a type of debris disk. ESO/L. Calcada

The birth or death of a planetary system is never anything less than spectacular – and, thanks to the marvels of modern astronomy, we can witness distant worlds and planetesimals begin and end around blazing stars in the deep, enigmatic dark. Often, the rotating debris disks of gas and dust whizzing around these spheres of light are beautiful, geometrically perfect circles.

Zoom in, though, and fine details begin to emerge, of remarkable arcs, strange indentations, and curious streaks; imperfections in the masterpieces taking shape. It’s previously been assumed that to create these idiosyncratic touches, you’d need a planet, whose gravity would alter the movement of the rings – but a new NASA study suggests something rather remarkable may also be happening.

According to simulations run on the Discover supercomputing cluster at NASA’s Goddard Space Flight Center, the intense light from high-energy stars can disrupt these distant disks, which etches stunning, ephemeral patterns into them. In order to understand how, we need to go back to 2013.

Back then, Wladimir Lyra, a professor of astronomy at California State University, Northridge, and Marc Kuchner, an astrophysicist at Goddard, teamed up to better understand why disks aren’t always perfectly formed. Publishing a new model in Nature, they explained that sharply defined rings and broken circles – or arcs – could form when stars emit enough ultraviolet light.

Ultraviolet light is incredibly energetic, so when it smashes into the gas, dust, and ice within these disks, it can strip away electrons from their individual particles. Although some may be ejected into deep space, others will cascade into other parts of the disk, which will trigger heating.

Heat the gas, and you amp up its pressure, causing it to expand. This attracts more dust, which often warms up nearby gas. Thanks to this so-called “photoelectric instability” (PeI) mechanism, a self-sustaining cycle begins, and strange shapes within the disk take shape.

Exoplanet hunters sometimes look for these weird clumps in an attempt to indirectly detect them, but the key finding of this 2013 study was that you don’t need planets to make these patterns.

What this new study – spearheaded by Penn State doctoral student Alexander Richert – does is enhance this original model. As it turns out, aside from PeI, there’s another destructive artist at work here: the pressure of starlight.

Electromagnetic radiation of any kind causes pressure changes whenever it passes through a medium. As photons bounce off the surface of matter they encounter, they transfer their momentum to it, like a ghostly hand pushing down on a table. The pressure increase is incredibly small per photon, but if you have enough of them, it can add up.

Highly energetic stars near debris disks, then, also exert a not-insignificant radiation pressure on those clumps of gas and dust, which alters their shape. As the team explain in their study – not yet peer-reviewed, but available here as a pre-print – this can mold debris disks into spiral patterns.

Along with Pel, these two mechanisms neatly explain the unique forms of many disks spotted over the last few years, all without needing to invoke planetary action.

The story likely won’t end here. There are other ways to make debris disks unique, and the team suggest a few.

An artist's impression of a snow line around a young star, V883 Orionis. A. Angelich/ESO

One rather marvelous idea, first discovered in 1959, is that gases containing free electrical charges can be influenced by regional magnetic fields. This “magnetorotational instability” or MRI can cause the disk to become somewhat unstable, and stretch out, creating endless new forms.

In any case, this new study reminds us that there’s so much we have yet to discover about the theatrics of the universe. In time, we’re sure that other cosmic ballets, hidden for billions of years, will be uncovered by work like this.

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